(1) Field of the Invention
The present invention concerns improvements in membrane tanks specially designed for the storage and transport of liquids at low temperatures, such as liquified gases.
The invention is of special applicability in the transport of liquified gases aboard ships.
(2) Description of the Prior Art
The transport of liquified gases aboard ships, for instance natural gas, is acquiring more importance every day due to the increase in the consumption of these gases, as well as the fact that the places or countries which consume the most of it, do not, in general, coincide with the producing zones or countries.
The transport and storage of natural gas or other gases in liquid state poses big problems, due to the low temperatures at which it is transported. These temperatures are usually around -162.degree. C.
Such low temperatures cause contractions in the walls of the tanks which lead to strong tensions.
In case of transport aboard ships, besides the thermal problems already pointed out, others, of mechanical and safety nature must be added.
The mechanical problems derive from vibrations, flexions, change of pressure, and, in general, from the movements of the ship, which expose the walls of the tanks to considerable static and dynamic effects.
The safety problems are due to the demands of the international organizations involved in classification, which demand a second waterproof barrier able to retain the possible leaks of the tank, so that the leaked liquid cannot enter in contact with the structure of the ship, which being basically made of ordinary steel, would not be able to endure the low temperatures of the liquid.
Presently, for the storage and transport of liquified gases, two types of tanks are essentially used -- self-carrying tanks, and integral or membrane tanks which are made of special alloys of steels or aluminum.
In the self-carrying tanks, the walls are made of high thickness, able to resist the thermal tensions, as well as the static and dynamic effects due to the thermal and mechanical stresses mentioned before.
With the integral or membrane tanks, however, the walls are of reduced thickness having the sole function of a watertight or curb barrier. In this type of tank, the wall lacks it own rigidity passing onto the outer carrying structure all static and dynamic pressures. Thermal tensions are avoided, in these tanks, by building the walls with special alloys, such as INVAR, of low dilatation factor, or, with steels with less nickel content, thus making on the wall undulations or corrugations conveniently spread out, which would act as bellows able to absorb the contractions and dilatations originated by the changes of temperatures.
In any event, the tanks would be covered with an insulating material. When the tanks are made of membrane, their walls are supported on the insulating coating, this being of enough rigidity to transmit the static and dynamic pressures onto the carrying structure.
Regardless of whether the tanks are of the self-carrying or membrane type, their construction, secondary and insulating barrier, as well as their installation, must be made separately, all of which should be considered in the final cost of the whole structure.